The subject matter of the invention is a sewing or embroidery machine, and in particular to sewing or embroidery machines with sensors associated with the lower thread supply.
When sewing and embroidering with a sewing machine, it is known to loop two threads, the upper thread and the lower thread, with each other. The upper thread, also called the needle thread, is supplied from a spool, which is arranged on or near the sewing machine and which is freely accessible and exchangeable. Its size can be selected essentially freely. The lower thread is wound on a lower-thread bobbin, which is placed in the interior of the sewing machine hook, which is supported and driven so that it can rotate, with the bobbin being supported in the hook so that it can rotate freely. Consequently, the diameter of the hook also determines the maximum size or diameter of the lower-thread bobbin lying therein. The quantity of lower thread wound onto the lower-thread bobbin, i.e., the lower-thread supply, is always smaller by a multiple in comparison with the upper-thread supply on the spool arranged outside of the machine housing. In addition, the lower-thread bobbin cannot be seen from the outside while sewing, because it is located inside the hook housing and the latter is located inside the housing of the sewing machine. For this reason, monitoring the current lower-thread supply and the end of the thread is difficult during the embroidery or sewing process and is associated with technical expense.
From the state of the art, measurement devices are already known, with which attempts have been made to determine the remaining quantity of lower thread on the lower-thread bobbin as exactly as possible and to stop the sewing machine before the end of the lower-thread is pulled out through the material being sewn by the upper-thread and before stitches are sewn, which are consequently not held by a lower-thread on the bottom side of the material being sewn.
From DE-C2 34 47 138, such a device is known on a two-step lock-stitch sewing machine, with which the winding of the lower-thread bobbin and the sewing operation can be monitored. There are bore holes in the lower-thread bobbin in the front flange at a constant radial distance to the bobbin rotational axis, i.e., on a common reference circle. Light beams from a light source are guided through the thread space of the bobbin between the circular ring-shaped flanges of the bobbin body to the rear flange and reflected from the rear flange for a low thread supply and detected by a light beam receiver. This device can determine a remaining thread quantity when its diameter on the bobbin becomes smaller than the diameter, at which the bore holes are located. In addition, through the intermittent reflection created by the spaced bore holes, it can be determined whether the bobbin is stationary (thread break or end of thread) or whether the bobbin is still rotating (driven by the thread pull). Thus, this device can determine when the thread quantity falls below a minimum value and the rotational state of the bobbin. However, it is not possible to determine the decrease in thread quantity per unit of time on the bobbin and consequently it is also not possible to calculate from this the expected time to the end of the thread. There is also no way to determine the thread thickness.